1. Field of the Invention
This invention relates to the fluidized catalytic cracking (FCC) conversion of heavy hydrocarbons into lighter hydrocarbons with a fluidized stream of catalyst particles and regeneration of the catalyst particles to remove coke which acts to deactivate the catalyst. More specifically, this invention relates to cracking of FCC feedstreams in a transport contacting conduit.
2. Description of the Prior Art
Catalytic cracking is accomplished by contacting hydrocarbons in a reaction zone with a catalyst composed of finely divided particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds, substantial amounts of coke are deposited on the catalyst. A high temperature regeneration within a regeneration zone operation burns coke from the catalyst. Coke-containing catalyst, referred to generally by those skilled in the art as xe2x80x9cspent catalystxe2x80x9d, is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone. Methods for cracking hydrocarbons in a fluidized stream of catalyst, transporting catalyst between reaction and regeneration zones, and combusting coke in the regenerator are well known by those skilled in the art of FCC processes. To this end, the art is replete with vessel configurations for contacting catalyst particles with feed and regeneration gas, respectively.
Despite the long existence of the FCC process, techniques are continually sought for improving product recovery both in terms of product quantity and composition, i.e. yield and selectivity. One facet of the FCC process that receives continued attention is the initial contacting of the FCC feed with the regenerated catalyst. Improvement in the initial feed and catalyst contacting tends to benefit yield and selectivity.
A variety of devices and piping arrangements have been employed to initially contact catalyst with feed. Most recent FCC arrangements contact catalyst in a riser conduit that transports the feed and catalyst upwardly in dilute phase as the reaction occurs. U.S. Pat. No. 5,017,343 is representative of devices that attempt to improve feed and catalyst contacting by maximizing feed dispersion. Another approach to improved feed and catalyst contacting is to increase the penetration of the feed into a flowing stream of catalyst. U.S. Pat. No. 4,960,503 exemplifies this approach where a plurality of nozzles surround an FCC riser to shoot feed into a moving catalyst stream from a multiplicity of discharge points. While these methods do improve distribution of the feed into the hot regenerated catalyst stream, there is still a transitory period of poor distribution when the relatively small quantities of the hydrocarbon feed disproportionately contact large quantities of hot catalyst. This poor thermal distribution results in non-selective cracking and the production of low value products such as dry gas.
The processing of increasingly heavier feeds and the tendency of such feeds to elevate coke production and yield undesirable products has led to new methods of contacting FCC feeds with catalyst. Of particular interest recently have been methods of contacting FCC catalyst for very short contact periods. U.S. Pat. No. 4,985,136 discloses an ultrashort contact time process for fluidized catalytic cracking, the contents of which are hereby incorporated by reference, that contacts an FCC feed with a falling curtain of catalyst for a contact time of less than 1 second and follows the contacting with a quick separation. U.S. Pat. No. 5,296,131, the contents of which are hereby incorporated by reference, discloses a similar ultrashort contact time process that uses an alternate falling catalyst curtain and separation arrangement. The ultrashort contact time system improves selectivity to gasoline while decreasing coke and dry gas production by using high activity catalyst that contacts the feed for a relatively short period of time. The inventions that provide short contact time are specifically directed to zeolite catalysts having high activity. The short contact time arrangements permit the use of much higher zeolite content catalysts that increase the usual 25-30% zeolite contents of the FCC catalyst to amounts as high as 40-60% zeolite in the cracking catalyst. These references teach that shorter hydrocarbon and catalyst contact time is compensated for by higher catalyst activity. Methods for ultrashort catalyst and feed contacting require unconventional contacting equipment and extensive replacement of existing equipment.
Many methods of ultrashort catalyst contacting perform an initial fast separation of the primary reacted products and collect the catalyst in a dense bed. The catalyst that enters the dense bed still contains a large amount of adsorbed and entrained hydrocarbons. The continued contacting of these hydrocarbons in a dense phase catalyst bed leads to overcracking of the remaining hydrocarbons and results in loss of products and the production of unwanted light gases.
The mixing of additional spent catalyst with the carbonized catalyst or the addition of catalyst to a traditional FCC riser arrangement or non-traditional short contact time arrangements have also been advantageously employed. U.S. Pat. No. 5,451,313 issued to Wegerer is an arrangement wherein regenerated and spent catalyst are mixed in a distinct chamber at the bottom of the riser and a secondary product stream is withdrawn from the riser. U.S. Pat. No. 5,858,207 issued to Lomas teaches the mixing of spent and regenerated catalyst at the bottom of the riser. The mixing of the regenerated and spent catalyst offers advantages of varying catalyst-to-oil ratios without the increase in catalyst temperature that occurs by the use of regenerated catalyst alone. In this regard, spent catalyst has been found to have sufficient activity to be particularly useful in providing a blended. catalyst mixture.
Therefore, improved or alternate methods are sought for ultrashort catalyst contacting. Improved methods will contact the feed using more conventional type equipment and with more traditional operations. Other improvements will focus on the better control of entrained and adsorbed hydrocarbons that are left on the catalyst.
It is an object of this invention to improve the control of cracking reaction time for light readily cracked hydrocarbons and more refractory heavy hydrocarbons that are adsorbed or otherwise entrained with catalyst.
Another object of this invention is to provide initial ultrashort contacting of the feedstream in a transport conduit with continued controlled residence time cracking of adsorbed or entrained hydrocarbons that remain entrained with the catalyst after withdrawal of the initial product.
A further object of this arrangement is to provide a short contact time system that can be readily operated to provide more traditional contact times.
This invention is an FCC process arrangement that uses a conventional FCC transport contacting conduit to contact feeds for reduced periods of time before initial withdrawal of a product followed by continued cracking of additional hydrocarbons within the transport contacting conduit. This arrangement can reduce the contacting time for initial contact between an FCC feedstream and catalyst in a transport conduit type reaction zone to times similar to those of other ultrashort feed and contacting arrangements. By recovering an initial product stream from an intermediate section of the transport conduit, rapidly cracked products are quickly recovered without stopping the continued flow of remaining reactants and products through the contacting conduit. The remaining hydrocarbons that are entrained or adsorbed onto the catalyst that passes the first product withdrawal section undergo further cracking through the conduit which can be controlled by varying the length or velocity through the remainder of the conduit.
The arrangement is susceptible to a large number of variations. The separation section in the intermediate section of the riser can be any type of separation that will perform an at least partial separation of gas phase materials from the catalyst without stopping or extensively disrupting the continued flow of catalyst and hydrocarbons through the conduit. Whatever separation devices are provided, it need not provide a complete separation of catalyst from gases, but will preferably provide enough separation to create an initial product stream that is primarily gas phase. Additional catalyst may be recovered from the intermediate product stream through any form of additional separator. Catalyst recovered from the gas stream may be returned to the process for stripping, regeneration, or preferably for recycle to the upstream end of the contacting conduit.
The arrangement of this invention may also have more than one intermediate withdrawal point. Additional withdrawal points may be spaced up the riser to obtain a variety of rough product fractions.
The section of the contacting conduit downstream of the intermediate product withdrawal point or any additional withdrawal point can be operated as a separate reaction section. To this end, additional feeds may be added downstream of any intermediate product stream withdrawal point. Preferred feeds for secondary products will comprise light cycle oil, heavy cycle oil, heavy oil, and heavy naphtha.
In order to maximize residence time control at the end of the transport conduit, it will preferably use a highly contained separation system that again provides a rapid separation from catalyst and gases to rigorously control residence time downstream of the initial product withdrawal. A large number of highly contained separation systems are known for use at the end of riser conduits such as direct connected cyclones and low volume containment vessels that surround the end of the riser, and containment devices that tangentially discharge the catalyst from the end of the riser.
The invention can also use any arrangement of transport conduit for the contacting of the catalyst and feed. Traditional FCC arrangements have used an upward transport riser where catalyst and gases are transported upwardly through the riser and withdrawn from the upper end of the riser. Downflow transport conduits, wherein catalyst is charged to an upstream end of the conduit, have been increasingly proposed. In such arrangements, the contacting takes place as gas transports the catalyst downwardly through the conduit with the added assistance of gravity. This invention may be advantageously employed to a transport conduit having a variety of shapes and directional orientations. However, it is most advantageously employed to either an upflow riser or a downflow conduit.
Accordingly, in a broad embodiment, this invention is a process for the fluidized catalytic cracking of a hydrocarbon stream. The process passes a first stream of catalyst particles comprising regenerated catalyst to a transport contacting conduit. A fresh feedstream contacts catalyst particles in the conduit which transports a mixture of the feedstream and the catalyst therethrough. The mixture of catalyst and feed passes through a first stage of separation located in an intermediate section of the conduit while maintaining continuous fluid flow of at least a portion of the mixture through the intermediate section of the conduit while withdrawing a separated portion of the mixture from the intermediate section of the conduit. Separation produces a lower catalyst density in the portion of the mixture withdrawn from the intermediate section. The remainder of the mixture continues downstream through the conduit to at least one second stage of separation for withdrawing a second mixture from the conduit that at least contains gas phase components. At least a portion of the spent catalyst withdrawn downstream of the first stage of separation passes to a regenerator section that regenerates the spent catalyst to provide the regenerated catalyst.
In a more limited process embodiment, this invention is a process for the fluidized catalytic cracking of a hydrocarbon-containing stream. The process blends a mixture of carbonized and regenerated catalyst at the bottom of the riser conduit to produce a blended catalyst mixture. The blended catalyst mixture contacts a feedstream in the conduit and passes up a first section of the riser to a ballistic separation device that separates a substantially gas phase stream from the feedstream and catalyst mixture. The substantially gas phase stream passes to a separator to recover a product stream and carbonized catalyst. At least a portion of the carbonized catalyst flows back to the bottom of the riser for blending with regenerated catalyst. The remainder of the feedstream and catalyst mixture continues downstream through a second section of the riser and at least partially continuous flow path. The remainder of the feedstream and catalyst mixture is withdrawn from a downstream end of the riser and is separated into a second product stream and a spent catalyst stream. The spent catalyst stream passes to a regenerator to provide the regenerated catalyst.
In an apparatus embodiment, this invention is an apparatus for the fluidized catalytic cracking of hydrocarbons. The apparatus has a transport conduit. The transport conduit is divided into at least three sections. The first section defines a catalyst inlet in communication with a source of regenerated catalyst near its upstream end. Means are provided for injecting a feedstream into the first riser section downstream of the catalyst inlet. A second section is in the path of direct gas and catalyst flow from the first section and defines a short contact product outlet. A third section of the riser is in the path of direct gas and catalyst flow from the second section and defines a secondary product outlet at its downstream end. A secondary product separator communicates with the secondary product outlet for separating spent catalyst from the secondary product. A stripper section strips hydrocarbons from the spent catalyst to produce stripped catalyst. A regenerator removes coke from the stripped catalyst to provide the source of regenerated catalyst.
The blending of carbonized and regenerated catalyst can provide ancillary advantages to the process. Combining both regenerated and carbonized catalyst in the ultrashort contacting zone and the disengaging vessel increases the solids-to-feed ratio in the reaction zone. A greater solids ratio improves catalyst and feed contacting and since the carbonized catalyst still has activity, the catalyst-to-oil ratio is increased. Moreover, the larger quantity of catalyst more evenly and quickly distributes the heat to the feed. The term xe2x80x9ccarbonized catalystxe2x80x9d refers to regenerated catalyst that has had at least some contact with the feed to deposit coke on the catalyst. Carbonized catalyst is usually referred to as xe2x80x9cspent catalystxe2x80x9d. However, spent catalyst is often thought of as originating from an FCC stripper. Accordingly, the term xe2x80x9ccarbonized catalystxe2x80x9d has been used in this application since the source of the carbonized catalyst can be from the intermediate section of the reaction conduit and may or may not include stripping.
The presence of coke on the catalyst can also benefit the process by reducing undesirable catalytic cracking reactions. The undesirable bimolecular reactions occur at highly acidic sites on the catalyst that are present on the fully regenerated catalyst. These sites strongly attract the hydrocarbon and are rapidly deactivated by coke accumulation. As subsequent recirculation passes coked particles through multiple cycles of riser contact without regeneration, these non-selective sites remain covered with catalyst so that only the more selective cracking sites remain active on the catalyst. The circulation of more selective sites can improve the yield of more desirable products.
The blending of catalyst is particularly suited for short contact time reaction systems and can be particularly useful in this invention. Under short contact time conditions, the catalyst and feed are kept in contact for very short periods of time and then quickly separated such that the catalyst undergoes little deactivation. Therefore, this invention will facilitate the recirculation of carbonized catalyst to the reaction zone without regeneration. The more feed and contact times are reduced, less deactivation will occur on the catalyst particles. The recycle of carbonized catalyst back to the riser also provides a convenient place for the return the catalyst separated in the intermediate section.
Additional objects, embodiments, details, and alternate arrangements for this invention are described in the following detailed description of the invention.